I read that if you use your credit cards, you build credit. I also read that if you get credit cards, your creditworthiness suffers. Both of these seem to make sense. Doing the former means that you show that you will pay debt back. It also makes sense to me that each person has some total amount of money they can be trusted to pay back at a fixed risk and if that person can use up more of that amount by simply using their credit cards, there is less left of other loans.
However, when I actually got a copy of the personal information stored by Schufa (I live in Germany), they neither gave me any information on how much I use my credit cards nor what the credit limits of my credit cards are. As I understand the GDPR, they need to give me all information they have about me and how big of a loan I'm trusted by what credit card company certainly sounds like information about me.